A boiling water reactor vessel is constructed in such a manner that it has a relatively large underwater nozzle extending through its sidewall which is used to recirculate water in the core shroud. This nozzle is relatively large--conventionally, for example, 28" in diameter. Quite often the piping and valving associated with the line leaving this nozzle require servicing, or even replacement. When such service is required, it would be desirable to be able to perform it without having to drain the reactor pressure vessel. In this manner, the necessity for removing the reactor fuel to a storage pool could be avoided. A smaller sized nozzle can often be plugged by means of a "freeze seal". This is a technique wherein liquid nitrogen is circulated around the nozzle to freeze the water within to effectuate a seal. However, the usual recirculating nozzle is too large to employ such a technique.
In the past, it has been possible to plug the recirculation nozzle of a boiling water reactor vessel by lowering into the vessel a disk shaped plug which is curved to fit against the vessel sidewall and includes a relatively deep, conical extension which serves to position the plug in the nozzle opening. A hydraulic cylinder on the rear of the plug pushes against the core shroud to seat the plug in the nozzle opening. A circular pneumatic sealing member on the edge of the plug disk is then inflated to seal off the plug.
The foregoing technique has been successfully applied in older-type reactor vessels. However, the plug which is used in that technique is relatively thick, approximately 61/2 inches. Nevertheless, it could be lowered straight down in the annular space between the core shroud and the reactor wall. In the newer reactors, however, there have been added a number of elements which form obstructions, thereby preventing the relatively thick prior art plugs from being employed. The primary obstructions are jet pumps which are positioned in the space between the reactor wall and the core shroud. Prior art plugs are too thick to pass between the jet pumps and the sidewall. Even if they could, they would be prevented from doing so because of intervening plumbing, such as core spray inlet pipes and feedwater spargers, and also by the reactor pressure vessel guide rod.
Accordingly, it is a primary object of the present invention to provide improved method and apparatus for plugging the recirculation nozzle of a water-filled boiling water reactor pressure vessel. Other objects, features, and advantages will be apparent from the following description and appended claims.